Compound mechanism and formulation design of main and auxiliary antioxidants against thermal oxygen aging
The anti-thermal oxygen aging of the polymer is mainly achieved by adding antioxidants, which can be divided into two kinds of primary antioxidants and auxiliary antioxidants according to their mechanism of action, and the two are used in combination, which has a synergistic effect and plays a better anti-thermal oxygen aging effect.
- Mechanism of action of primary antioxidants
The main antioxidant can react with free radicals R· and ROO·, capture and remove active free radicals, convert them into hydroperoxides, interrupt the growth of the active chain, eliminate the free radicals generated by the resin under high temperature, heat and light conditions, and achieve the purpose of protecting the polymer. The specific mode of action is as follows:
Hydrogen donors, secondary arylamines and hindered phenolic antioxidants contain -OH, = NH groups, which can provide hydrogen atoms to free radicals, so that active radicals generate stable radicals or hydroperoxides.
Free radical traps, benzoquinone antioxidants react with free radicals to form stable free radicals.
Electron donor, tertiary amine antioxidants provide electrons to reactive radicals, making them low-activity negative ions, terminating auto-oxidation reactions.
Primary antioxidants can be used alone, but work better with secondary antioxidants.
- Mechanism of action of auxiliary antioxidants
Auxiliary antioxidants can decompose hydroperoxides generated by the primary antioxidant that still have some activity, so that they do not re-initiate the automatic oxidation reaction.
In addition, auxiliary antioxidants can inhibit and delay the formation of free radicals during the initiation process, and passivate the metal ions remaining in the polymer. Auxiliary antioxidants such as phosphite esters and organic sulfides are hydroperoxide decomposing agents.
- Selection of antioxidants
There are many varieties of antioxidants, and the following points should be paid attention to when choosing.
(1) Compatibility, compatibility refers to the fusion performance of antioxidants and resins within the dosage range, and the compatibility of commonly used hindered phenols and phosphite esters with PE is good.
(2) Processing performance, after the addition of antioxidants to the resin, the melt viscosity and the torque of the screw may change, such as the melting point of the antioxidant and the resin is very different, but also may produce screw and deflection phenomenon, for this reason generally do not choose antioxidant varieties with melting points lower than 100 °C.
(3) Polluting and hygienic, amine antioxidants are an excellent class of primary antioxidants with high antioxidant efficiency. However, it will change color during processing and contaminate the product, and the toxicity is large, so it is generally not used in polymer products that require hygiene.
(4) Stability, amine antioxidants will change color under the action of light and oxygen, antioxidant BHT is easy to volatile decomposition during processing, phosphite esters are easy to hydrolyze, hindered amines are heated in acidic substances, and dehydrogenation reaction will occur. All of the above will affect the antioxidant effect.
(5) Extraction resistance and volatility, extraction resistance refers to the ease of dissolving the antioxidant in the product in contact with the liquid, the greater the relative molecular mass of the antioxidant, the more difficult it is to extract. Volatile refers to the phenomenon that polymer products containing antioxidants escape products when heated, and the higher the melting point and the greater the relative molecular weight, the volatility of antioxidants is small.
- Selection of primary antioxidants
Hindered phenolic primary antioxidant is most commonly used in polymers because it does not contaminate the product, is close to white, non-toxic or low toxicity. The addition amount of 0.4%~0.45% hindered amine main antioxidant has good antioxidant, but it is easy to color and toxic polymer products, and it is less used in polymers. Sometimes it can only be used in dark polymer products. The synergistic addition of different varieties of primary antioxidants has better effect than single addition, such as hindered phenol/hindered phenol or hindered amine/hindered phenol combination.
- Selection of auxiliary antioxidants
Phosphite has a good synergistic effect with the main antioxidant, and has a certain degree of antioxidant, heat resistance, weather resistance and color are good, is a commonly used auxiliary antioxidant, the disadvantage is poor water resistance, but can choose the newly developed water-resistant type. The application of sulfur-containing compound auxiliary antioxidants is not as extensive as phosphites, and it is easy to produce sulfur pollution when combined with some additives, and has a counter-effect with HALS light stabilizers.
- Synergistic effect of primary and auxiliary antioxidants
Auxiliary antioxidants must be added in synergy with the primary antioxidant to have antioxidant effect, and can reduce the amount of primary antioxidant added, and its addition alone has no antioxidant effect. The composite types of antioxidants are hindered phenol/thioether, phosphite/hindered phenol, etc. The main antioxidant is phenolic 1010, 1076, 264, etc., and the secondary antioxidant is phosphite 168.
Post time: Nov-30-2022